An enzyme-linked immunosorbent assay, or ELISA, test is a type of medical diagnostic test used to detect whether a certain antibody or antigen is present in a patient. It can be useful for a range of different purposes relating to immunology, such as disease testing and virus testing. For example, an ELISA HIV test may be administered to determine whether a patient has been infected with HIV antibodies. In addition, ELISA tests are sometimes also used in testing for drug use. An ELISA test can also help detect allergic reactions to food products like nuts or dairy items.
During an ELISA procedure, a health care provider typically collects a blood sample from a patient. This is usually done by inserting a needle into a vein on the back of the patient’s hand or in the patient’s inner elbow area. Alternatively, an ELISA test may allow for a urine sample. In any case, the collected test sample is placed into a test tube or onto a test slide or strip. The health care provider then sends the sample to a laboratory for analysis.
At the laboratory, technicians will determine whether the targeted antibody or antigen is present in the test sample. If a patient has a certain disease or condition, his or her sample will contain antibodies for that disease or condition. These antibodies will latch on to antigens, which are used as bonding agents in most ELISA tests.
The lab technician will clean the test sample using a special test solution that washes away everything but the antigens, or the antibodies that cling to the antigens. Next, the lab technician applies an enzyme solution to the test sample. If the sample changes colors or provides some other indication, the target antibody or antigen is present in the test sample and the patient will test positively for the condition.
In general, an ELISA test is considered reliable within the immunology community. It is possible, however, for a patient who does not actually have the targeted infection to experience a phenomenon known as a false positive. A false positive occurs when a patient who is not infected with the target antibodies gives a positive result during the ELISA test.
False positives can occur for several reasons. For instance, if a sample becomes contaminated or inadvertently switched in the laboratory, a false positive may result. Patients with hemophilia or hemodialysis, or alcoholic patients with hepatitis are also more susceptible to experiencing false positives. Injection drug users and women who have had multiple pregnancies can also be more likely to realize false positives.
There are variations of the ELISA test, but the most basic type consists of an antibody attached to a solid surface. This antibody has affinity for (will latch on to) the substance of interest, for example, human chorionic gonadotropin (HCG), the commonly measured protein which indicates pregnancy. A mixture of purified HCG linked (coupled) to an enzyme and the test sample (blood, urine, etc) are added to the test system. If no HCG is present in the test sample, then only HCG with linked enzyme will bind. The more HCG which is present in the test sample, the less enzyme linked HCG will bind. The substance the enzyme acts on is then added, and the amount of product measured in some way, such as a change in color of the solution.
ELISA tests are generally relatively accurate tests. They are considered highly sensitive and specific and compare favorably with other methods used to detect substances in the body, such as radioimmune assay (RIA) tests. They have the added advantages of not needing radioisotopes (radioactive substances) or a costly radiation counter (a radiation-counting apparatus).
Before the development of the EIA/ELISA, the only option for conducting an immunoassay was radioimmunoassay, a technique using radioactively-labeled antigens or antibodies. In radioimmunoassay, the radioactivity provides the signal which indicates whether a specific antigen or antibody is present in the sample. Radioimmunoassay was first described in a paper by Rosalyn Sussman Yalow and Solomon Berson published in 1960.
Because radioactivity poses a potential health threat, a safer alternative was sought. A suitable alternative to radioimmunoassay would substitute a non-radioactive signal in place of the radioactive signal. When enzymes (such as peroxidase) react with appropriate substrates (such as ABTS or 3,3’,5,5’-Tetramethylbenzidine), this causes a change in color, which is used as a signal. However, the signal has to be associated with the presence of antibody or antigen, which is why the enzyme has to be linked to an appropriate antibody. This linking process was independently developed by Stratis Avrameas and G.B. Pierce. Since it is necessary to remove any unbound antibody or antigen by washing, the antibody or antigen has to be fixed to the surface of the container, i.e. the immunosorbent has to be prepared. A technique to accomplish this was published by Wide and Porath in 1966.
In 1971, Peter Perlmann and Eva Engvall at Stockholm University in Sweden, and Anton Schuurs and Bauke van Weemen in The Netherlands, independently published papers which synthesized this knowledge into methods to perform EIA/ELISA.